A major source of methane is natural gas which typically contains about 40-95% methane depending on the particular source. Other constituents include about 10% of ethane with the balance being made up of CO.sub.2 and smaller amounts of propane, the butanes, the pentanes, nitrogen, etc.
Primary sources for natural gas are the porous reservoirs generally associated with crude oil reserves. From these sources come most of the natural gas used for heating purposes. Quantities of natural gas are also known to be present in coal deposits and are by-products of crude oil refinery processes and bacterial decomposition of organic matter. Natural gas obtained from these sources is generally utilized as a fuel at the site.
Prior to commercial use, natural gas must be processed to remove water vapor, condensible hydrocarbons and inert or poisonous constituents. Condensible hydrocarbons are generally removed by cooling natural gas to a low temperature and then washing the natural gas with a cold hydrocarbon liquid to absorb the condensible hydrocarbons. The condensible hydrocarbons are typically ethane and heavier hydrocarbons. This gas processing can occur at the wellhead or at a central processing station. Processed natural gas typically comprises a major amount of methane, and minor amounts of ethane, propane, the butanes, the pentanes, carbon dioxide and nitrogen. Generally, processed natural gas comprises from about 70% to more than about 95% by volume of methane. Natural gas is used principally as a source of heat in residential, commercial and industrial service.
Most processed natural gas is distributed through extensive pipeline distribution networks. As natural gas reserves in close proximity to gas usage decrease, new sources that are more distant require additional transportation. Many of these distant sources are not, however, amenable to transport by pipeline. For example, sources that are located in areas requiring economically unfeasible pipeline networks or in areas requiring transport across large bodies of water are not amenable to transport by pipeline. This problem has been addressed in several ways. One such solution has been to build a production facility at the site of the natural gas deposit to manufacture one specific product. This approach is limited as the natural gas can be used only for one product, preempting other feasible uses. Another approach has been to liquefy the natural gas and transport the liquid natural gas in specially designed tanker ships. Natural gas can be reduced to 1/600th of the volume occupied in the gaseous state by such cryogenic processing, and with proper procedures, safely stored or transported. These processes, which involve liquefying natural gas to a temperature of about -162.degree. C., transporting the gas, and revaporizing it are complex and energy intensive.
Still another approach has been the conversion of natural gas to higher molecular weight hydrocarbons that can be easily handled and transported, preferably substantially liquid hydrocarbons. The conversion of natural gas to higher order hydrocarbons, especially ethane and ethylene, would retain the material's versatility for use as precursor materials in chemical processing. Known dehydrogenation and polymerization processes are available for the further conversion of ethane and ethylene to liquid hydrocarbons. In these ways, easily transportable commodities may be derived directly from natural gas at the wellhead. A drawback in implementing such processes has been in obtaining a sufficient conversion rate of natural gas to higher molecular weight hydrocarbons.
The conversion of methane to higher molecular weight hydrocarbons at high temperatures, in excess of about 1000.degree. C. is known. These processes are, however, energy intensive and have not been developed to the point where high yields are obtained even with the use of catalysts. Some catalysts that are useful in these processes (e.g., chlorine) are corrosive under such operating conditions.
The catalytic oxidative coupling of methane at atmospheric pressure and temperatures of from about 500.degree. C. to 1000.degree. C. has been investigated by G. E. Keller and M. M. Bhasin. These researchers reported the synthesis of ethylene via oxidative coupling of methane over a wide variety of metal oxides supported on an alpha alumina structure in Journal of Catalysis, 73, 9-19 (1982). This article discloses the use of single component oxide catalysts that exhibited methane conversion to higher order hydrocarbons at rates no greater than 4%. The process by which Keller and Bhasin oxidized methane was cyclic, varying the feed composition between methane and nitrogen and air (oxygen) to obtain higher selectivities.
The conversion of methane to higher molecular weight hydrocarbons using metal oxide catalysts and oxides of carbon which are generated from the hydrocarbon is described in U.S. Pat. No. 2,180,672. The conversion generally is carried out at temperatures of from about 150.degree.-350.degree. C., and the oxides of carbon are consumed in the reaction.
Methods for converting methane to higher molecular weight hydrocarbons at temperatures in the range of about 500.degree. C. to about 1000.degree. C. are disclosed in U.S. Pat. Nos. 4,443,644; 4,443,645; 4,443,646; 4,443,647; 4,443,648; and 4,443,649. The processes taught by these references provide relatively high selectivities to higher order hydrocarbons but at relatively low conversion rates, on the order of less than about 4% overall conversion. In addition to synthesizing hydrocarbons, the processes disclosed in these references also produce a reduced metal oxide which must be frequently regenerated by contact with oxygen. The preferred processes taught by these references entail physically separate zones for a methane contacting step and for an oxygen contacting step, with the reaction promoter recirculating between the two zones.
U.S. Pat. Nos. 4,172,810; 4,205,194; and 4,239,658 disclose the production of hydrocarbons including ethylene, benzene, ethane, propane and the like, in the presence of a catalyst-reagent composition which comprises: (1) a Group VIII noble metal having an atomic number of 45 or greater, nickel, or a Group Ib noble metal having an atomic number of 47 or greater; (2) a Group VIb metal oxide which is capable of being reduced to a lower oxide; and (3) a Group IIa metal selected from the group consisting of magnesium and strontium composited with a passivated, spinel-coated refractory support or calcium composited with a passivated, non-zinc containing spinel-coated refractory support. The feed streams used in the processes disclosed in these patents do not contain oxygen. Oxygen is avoided for the purposes of avoiding the formation of coke in the catalyst. Oxygen is generated for the reaction from the catalyst; thus periodic regenerations of the catalysts are required.
U.S. Pat. No. 4,450,310 discloses a methane conversion process for the production of olefins and hydrogen comprising contacting methane in the absence of oxygen and in the absence of water at a reaction temperature of at least 500.degree. C. with a catalyst comprising the mixed oxides of a first metal selected from lithium, sodium, potassium, rubidium, cesium and mixtures thereof, a second metal selected from beryllium, magnesium, calcium, strontium, barium, and mixtures thereof, and optionally a promoter metal selected from copper, rhenium, tungsten, zirconium, rhodium, and mixtures thereof.
West German Patent DE 32370792 to Baerns and Hinsen describes the use of supported single component oxide catalyst. The process utilizes low oxygen partial pressure to give a high selectivity for the formation of ethane and ethylene. The conversion of methane to such desired products, however, is low, on the order of from about 4 to about 7% conversion.
U.S. Pat. No. 4,658,076 and Australian published Patent Application 8654350A describe catalyst systems which consist essentially of either Group IA metals/zinc oxide; or Group IA metals/zinc oxide/chloride ions, compounds containing chloride ions, tin, or compounds containing tin. The catalysts are reported as being useful in the oxidative conversion of feed organic compounds comprising methane to product organic compounds comprising higher hydrocarbons.
The oxidative coupling of methane to ethylene and ethane over lithium-promoted zinc oxide catalysts is described in an article by Matsuura et al, Chem. Letters, 1986, pp. 1981-84. Other alkali-promoted zinc oxides were also tested as catalysts for the oxidative coupling of methane, and of the alkali metals, lithium is reported to provide the best conversions and selectivity to ethane and ethylene.
The synthesis of ethylene with high selectivity and yield using a catalyst comprising the oxides of transition metals with lithium chloride has been reported by Otsuka et al, Chem. Letters, 1986, pp. 903-06. Of the transition metal oxides studied, manganese and nickel were reported to produce ethylene with the highest selectivity and yield.
European published Patent Application 198251 describes contact materials useful for the oxidative conversion of methane to higher hydrocarbons. The materials may be selected from the group consisting of (a) a component comprising at least one oxide of calcium, strontium or barium and, optionally, a component comprising chloride ions, compounds containing chloride ions, tin and compounds containing tin, or (b) a component comprising at least one metal from the group of sodium, potassium or compounds containing said metals, a component comprising at least one metal from Group IIA metals and compounds containing said metals, and optionally, chloride ions, compounds containing chloride ions, tin or tin compounds, or (c) a component comprising a Group IA metal and compounds containing said metal, a component comprising calcium, strontium, barium or compounds containing said metals and, optionally, chloride ions, compounds containing chloride ions, tin or compounds containing tin.
U.S. Pat. No. 3,119,883 describes ZnO promoted with 1 to 10 mole percent Pb, Bi, Sn or Fe and up to 1 weight percent alkali metal as an oxydehydrogenation catalyst for ethane conversion to ethylene.
U.S. Pat. Nos. 4,547,608 and 4,544,786 describe a method for converting methane to higher hydrocarbon products by contacting methane with a contact agent comprising (a) a reducible metal oxide such as lead oxide, (b) a support of at least two oxides such as the combination of an alkaline earth oxide with silica, alumina, or mixtures thereof, and (c) an alkali metal.
There continues to be a need for processes useful in converting light hydrocarbons such as methane and/or natural gas to higher molecular weight liquid hydrocarbons with high selectivity and conversion rates.